This invention relates generally to electronic article surveillance (EAS) systems and pertains more particularly to improved transmitters for generating magnetic fields for use in EAS systems.
Referring to FIG. 1, an EAS transmitter 10 placed in present commercial use by the assignee of the subject application comprises an oscillator 12 which generates a continuous output at 58 KHz, a rectangular pulse generator 14 which generates a rectangular pulse with a 1.6 millisecond ON time, a multiplier 16 receiving inputs from oscillator 12 and pulse generator 14, a linear amplifier 18 which amplifies the output of multiplier 16 and furnishes the amplified output to antenna 20 which radiates energy into a so-called surveillance zone (not shown) to establish a magnetic field in the surveillance zone. EAS tags (markers) entering the surveillance zone and not previously deactivated are activated by the magnetic field and are sensed by an EAS receiver which provides output alarm indication of the presence of the tags in the surveillance zone.
FIG. 2 is a frequency domain plot of the transmitted waveform of the FIG. 1 transmitter. As is seen, the magnetic field is at its highest strength at the carrier frequency of 58 KHz and has sidelobes at 2K aside the carrier frequency which are down only by approximately 20 dB from the 58 KHz fundamental level.
A prospective specification for EAS systems in Europe looks to a xe2x80x9cwide exitxe2x80x9d with a 60 KHz guardband. The 2K sidelobe strengths of transmitter 10, as illustrated in FIG. 2, only marginally meet the prospective specification.
The present invention has as its particular and immediate object the provision of an EAS transmitter meeting the guardband requirements of the prospective European EAS specification.
A more general objective of the invention is to provide method and system for conforming EAS transmitter waveforms to any desired fundamental and sidelobe magnitude relationship as may be required by EAS specifications.
A further object of the invention is to reduce energy in selected xe2x80x9ckeepoutxe2x80x9d bands of an EAS transmission waveform without loss of strength of the fundamental and without increasing the length of the transmit burst.
In attaining the foregoing and other objects, the invention provides an EAS transmitter for generating an EAS transmission with a preselected guardband, comprising: means for generating a fundamental frequency pulsed waveform; and means for effecting spectral window shaping of the fundamental frequency waveform on a time basis corresponding with the guardband for reducing sidelobe energy in the guardband.
The transmitter may further include means operative on the spectral window-shaped fundamental frequency waveform for further reducing sidelobe energy of the spectral window-shaped fundamental frequency waveform in frequency correspondence with the guardband.
The means for effecting spectral window shaping of the fundamental frequency waveform preferably includes means for generating a pulsed cosine waveform and means for multiplying the fundamental frequency waveform by the pulsed cosine waveform.
A method for effecting EAS transmission in accordance with the invention comprising the steps of: selecting a fundamental frequency pulsed waveform; identifying a guardband for the fundamental frequency pulsed waveform; and effecting spectral window shaping of the fundamental frequency waveform on a time basis corresponding with the identified guardband for reducing sidelobe energy in the identified guardband.
The method may include the further step of reducing sidelobe energy of the spectral window-shaped fundamental frequency waveform in frequency correspondence with the identified guardband.
The step of effecting spectral window shaping of the fundamental frequency waveform is performed in part through the use of a pulsed cosine waveform.
In a still further aspect, the invention contemplates an EAS transmitter in which a storage device stores a digitized transmission waveform derived from a device separable from the transmitter which inputs the digitized transmission waveform to the storage device. The digitized transmission waveform has a fundamental with a suppressed sidelobe.
These and other objects and features of the invention will be further understood from the following detailed description of preferred embodiments and practices thereof and from the drawings.